Bashkirian meganticlinorium: Late Riphean-Vendian hiatuses and possible transformations of basin provenances

Abstract

The object of research. At the top of the Upper Riphean-Vendian sedimentary sequence of the Bashkirian meganticlinorium (western and central parts of the named structure, corresponding to the stratotypical locality of Riphean) there is a series of fairly well-established hiatuses – the pre-Uk and pre-Bakeevo. One can also assume the existence of the pre-Uruk and pre-Basa hiatuses. During each of them, the thin-grained aluminosiliciclastic’s distribution systems could be subjected to one or another transformations. However, whether this is actually the case is still not entirely clear. To solve this problem, the lithogeochemical features of clayey rocks of the Inzer, Minyar, Uk, Bakeevo and Basa formations are analyzed. If we assume that the hiatuses are accompanied by restructuring of the provenances, then this can/should lead to the appearance of more fresh, previously not subject to weathering, rock complexes at the paleo-watersheds. This circumstance should cause a change in the lithochemical characteristics of the thin-grained aluminosiliciclastics entering to the sedimentation area. Methods. The analysis of the lithogeochemical features of clayey rocks is based on data on the content of the main rock-forming oxides (about 40 analyzes) and rare and trace elements (total of 70 analyzes). These data make it impossible to investigate the change in different lithogeochemical parameters of clayey rocks (for example, CIA, GM, K2O/Al2O3, or others) from the bottom upwards through the section of each of the formations listed above, therefore we operate with the averages for the formation values of such parameters taking into account the values their standard deviations. Results and conclusions. On the majority of the discriminant diagrams used by us, the figurative data points of the Basa Fm. mudstones occupy a position that to some extent differs from the position of the clayey rocks of other formations. The only lithostratigraphic subdivision in the part of the Upper Precambrian sedimentary sequence of the Bashkirian meganticlinorium under consideration, whose clayey rocks demonstrate a chemically noticeably less mature, compared to the underlying formations, is the Uk Fm. It may be thought that this is the result of the appearance of more fresh (less transformed by weathering) rock complexes at the provenances. The scales of the pre-Bakeevo and pre-Basa breaks, if they existed, were, most likely, noticeably less significant.